The machining and shaping of metal articles by means of milling and turning operations have been a part of modern society since the dawn of the Industrial Revolution. As would be expected, tools or at least the tips of tools for forming metal articles were initially fashioned from metals. As the feeding speeds and the rotating speeds in milling and turning operations increased, however, such that the tips of the tools encountered higher and higher temperatures, it soon became evident that the tips reacted chemically with the metal workpiece and quickly wore away. Inasmuch as those actions were undesirable, numerous efforts were undertaken to harden the tool tip, while decreasing the chemical reactivity thereof with respect to the metal workpiece.
As a result, the prior art is replete with materials for cutting tool tips (or "inserts" as defined in the cutting tool art) as substitutes for metals. In general, the prior art has disclosed the use of hard refractory ceramics as components for cutting tool inserts. To illustrate:
U.S. Pat. No. 4,063,908 describes the incorporation of TiO.sub.2 and TiC into an Al.sub.2 O.sub.3 sintered ceramic body. U.S. Pat. No. 4,204,873 reports the inclusion of WC and TiN in a sintered ceramic body containing Al.sub.2 O.sub.3. In like manner U.S. Pat. No. 4,366,254 records the addition of ZrO.sub.2, TiN or TiC, and rare earth metal carbides to a base Al.sub.2 O.sub.3 ceramic body.
In general, cutting tool inserts have been expressly designed for either milling or turning operations. That is, inserts designed for one operation have not customarily been used in the other because the wear characteristics of the two operations are quite different. Thus, cutting tool inserts designed for turning will commonly fail relatively rapidly when employed in a milling operation, with a like situation obtaining when tool inserts designed for milling are used in turning. More recently, cutting tool inserts are being produced which perform both turning and milling operations with limited success.
A variety of physical properties must be present for a ceramic cutting tool insert to perform satisfactorily. Among these properties are hardness, thermal conductivity, strength, and toughness (all as a function of temperature). Undesirable phase transformations of phases within the insert occurring with changes of temperature must be avoided and, as mentioned above, chemical reactivity with the workpiece should be minimized. Whereas an individual material may excel in several properties, a deficiency in another area may make the material useless as a cutting tool insert. An example of such a deficiency is zirconia, where the strength and toughness of the material are excellent but the thermal conductivity is low and the hardness is low. The low thermal conductivity property results in the tip of the insert during use becoming so hot that it can be made to flow plastically.
A standardized test has been developed for each of those two types of metal removal operations; viz., the turning test and the interrupted cut or milling test. The two tests can be broadly characterized in terms of the action each encounters. Hence, turning is largely a measure of an insert material's resistance to abrasion and chemical wear. The interrupted cut test measures the ability of an insert material to resist thermal and mechanical shock.
In the turning test, a bar of metal (the "workpiece") is mounted on a lathe and turned at predetermined speeds against the insert. The insert is mounted in a tool holder which is moved along the length of the workpiece. The amount of metal removed from the workpiece per unit time is a function of three factors: first, the speed at which the spindle that turns the workpiece rotates in terms of revolutions per minute (RPM); second, the rate at which the insert is moved from one end to the other parallel to its axis into the length of the workpiece by the tool holder, that rate being measured in terms of inches per minute per revolution (IPR) of the workpiece; and, third, the distance which the insert cuts into the workpiece, that distance being measured as the depth of cut (DOC). The first two operations combined give the standard measure for the rate of metal removal which is customarily defined in terms of surface feet per minute (SFPM). In the standard procedure for conducting the test, IPR is held at 0.010", DOC is maintained at 0.075", and the RPM is varied depending upon the desired rate of metal removal.
The interrupted cut test uses a turret lathe with a single insert mounted in the cutting head. As such, the insert essentially chops away at a workpiece as it is moved laterally across the rotating cutting head. The interrupted cut test is dynamic since the feed rate increases as the test progresses. In the test matrix of the present invention, the first twenty cuts are made with a feed rate of 0.0025 IPR which is increased after each successive 5 passes (or cuts) by 0.0025 IPR increments, so that on the twentieth pass the feed rate is 0.010 IPR. Subsequent cuts, 21-60, have an increased rate of 0.0050 IPR for each 5 passes, such that pass 21 has a feed rate of 0.015 IPR and cut 60 has a feed rate of 0.050 IPR. The feed rate of 0.050 IPR is the upper limit since it represents the maximum capacity of the test equipment. This test provides information regarding the resistance to thermal and mechanical shock of a material and is terminated at failure of the insert.
Good thermal and mechanical shock resistance is required for satisfactory performance of an insert in the milling operation. Additionally, such thermal and mechanical properties are required in turning operations. Under cutting conditions in turning operations, such as a heavy feed rate, deep depth of cut, or when a coolant is in use, an insert must have the ability to resist the thermal and mechanical force inherent to such conditions. The same durability must exist when the insert is subjected to an inhomogeneous workpiece material; for instance, where hard inclusions are encountered in the workpiece or when scaly surfaces are being turned down. Therefore, good performance in the interrupted cut screen test indicates that an insert material may perform well under conditions found in many turning operations.
The above tests can be designed to simulate accelerated wear tests by using increased cutting speeds. For example, the turning test employs speeds of about 2000-3000 SFPM, those rates being substantially higher than the 800-1000 SFPM typically used in industry. Thus, in general, the higher the cutting speed, the higher the temperature at the insert/workpiece interface. The elevated temperature (perhaps 1300.degree. C. or higher at 2500-3000 SFPM) at such high cutting speeds causes greater plastic deformation of the workpiece, thereby resulting in lower abrasive wear and mechanical shock due to cutting as the hot metal is removed. Higher temperatures, however, promote increased chemical reaction rates and, therefore, enhance temperature-related wear mechanisms; e.g., adhesive wear.
Whereas research has been extensive to develop improved inserts for cutting tools from ceramic compositions, there has remained the need for inserts designed for metal milling and turning operations which exhibit durability and reliability significantly better than products currently available.
Therefore, the primary objective of the present invention was to develop cutting tool inserts demonstrating exceptional toughness, wear resistance, impact resistance, thermal conductivity, and thermal shock resistance rendering them especially suitable for use in milling and turning operations.